Optical wave guide devices fabricated by the Proton Exchange (PE) method provide some unique qualities. The process of proton exchange increases the refractive index only in extraordinary axis and thus will only guide one polarization state. The other polarization state is unguided and is eventually eliminated. This quality of Proton Exchange devices makes them naturally very high performance polarizers (60 dB or more). In addition, this quality makes them very attractive for use in Multiple Function Chips (MFCs) used in construction of fiber optic gyros (FOGs).
Now refer to FIG. 1A which shows a schematic diagram of a proton exchange polarizer with crosstalk. A proton exchange polarizer 10 includes a LiNbO3 or LiTaO3 material forming an optically transmissive substrate 16. The proton exchange polarizer 10 further includes a glass ferrule 20 as an input coupling for an optical fiber 12 and a glass ferrule 22 coupled to an output fiber 14. The optical fiber 12 receives light comprised of both a TE mode 24 and a TM mode 26. The TE mode 24 includes the solutions of Maxwell's equations with symmetric boundary conditions for an optical wave propagating within the waveguide (i.e. LiNbO3 at both sides of the waveguide) and the TM mode 26 includes the solutions of Maxwell's equations with non-symmetric boundary conditions for an optical wave propagating within the waveguide (i.e. LiNbO3 at one side of the waveguide and air at the opposing side of the waveguide). TE mode light 32 is substantially guided by a proton exchange wave guide 18 through the optically transmissive substrate 16. When light exits the fiber 12 the TM mode 26 becomes unguided TM mode light 30. The TE mode 24 becomes guided TE mode light 32 by the proton exchange wave guide 18.
Unguided TM mode light 30 propagates through the optically transmissive substrate 16. A portion of the unguided TM mode light 30 from the polarizer escapes. An angle of reflection 28 may be determined by the dimensions of the particular embodiment, specifically the distance between the glass ferrules 20 and 22.
The TM mode unguided light 30 is reflected as indicated by light 36 from a bottom 17 and sides of the substrate 16 and exits the substrate through fiber 14. The reflected unguided TM mode light 36 is unwanted in various applications such as fiber-optic gyros.
During the development of proton exchange devices it was discovered that some devices do not have very high extinction ratios (more than 60 dB) as expected. It was also found that the polarizer extinction ratio decreased with device length. After studying different devices, it was concluded that the crosstalk/modulator mechanism had TM light 30 that was unguided by the wave guide 18 and reflected from the bottom 17 and sides of the substrate 16. The reflected TM light 36 was then collected by the output fiber 14. It was believed that longer polarizer length results in a smaller angle of reflection, which increases this pickup. Several polarizers of different lengths were measured to confirm this theory.